To circumvent the above difficulties, in this paper we propose a new integrated IRS-BS architecture, where four IRSs with different orientations and an antenna array are deployed within the same cuboid antenna radome at the BS, as shown in Fig. REF . Due to the ultra-short distance between the IRSs and BS's antenna array (i.e., in the order of several to tens of wavelengths), the path loss of the reflection channels via IRSs can be significantly reduced. In addition, besides the single-IRS reflections between the users and the antenna array of the BS, the double reflections among the IRSs become much stronger than their counterparts in the conventional double-IRS systems with IRSs deployed much farther away from each other [1]}, [2]}, and thus their effect to the system performance becomes significant and may even dominate over the single-IRS reflections. Moreover, the signaling overhead between the BS and IRSs is generally reduced because of their integrated architecture. Despite the above mentioned advantages, new technical issues also arise, which need to be investigated for our proposed integrated IRS-BS architecture. First, the conventional uniform plane wave (UPW) assumption cannot be directly applied to model the channels between the BS antenna array and each of IRSs as a whole since the far-field condition is not satisfied between them [3]}, [4]}, [5]}, which motivates us to develop a more practical and sophisticated channel model for the proposed integrated IRS-BS system. Second, the signals from/to the users experience single and double reflections by multiple integrated IRSs, and the coupling effect of these channel components makes the optimization of IRS passive reflection more intractable as compared to the conventional design considering single-IRS reflections only. Third, the CSI becomes more substantial and is harder to obtain in practice due to the additional inter-IRS reflections involved, despite various channel estimation methods have been proposed for single-IRS [6]}, [7]}, [8]}, [9]} or double-IRS systems [10]} (see [11]}, [12]}, and the reference therein). This is because our proposed architecture has multiple IRSs integrated very close to the BS's antenna array, and thus the number of channel coefficients to be estimated is much larger than that in conventional single-/double-IRS systems, which may require extremely high pilot overhead for estimating all reflection channels via all IRSs. To tackle the above challenges, we develop efficient algorithms for designing the IRS reflection coefficients under the case with/without perfect CSI at the BS, respectively. The main contributions of this paper are summarized as follows.
